One emerging technology for non-volatile memory is magnetoresistive random access memory (MRAM). At present, the most common form of MRAM is based on the tunneling magnetoresistance (TMR) effect, in which each memory cell includes a magnetic tunnel junction (MTJ). Such an MTJ may be formed from two ferromagnetic metal layers, with an insulating layer placed between the metal layers. When a voltage is applied between the metal layers, a tunnel current flows. The tunnel resistance varies based on the relative directions of magnetization of the metal layers. The tunnel resistance is small when the directions of magnetization are parallel (typically representing a “0”), and large (approximately 30%-300% higher, at room temperature) when the directions of magnetization are anti-parallel (typically representing a “1 ”).
The metal layers in a typical MRAM MTJ include a “hard” layer, in which the direction of the magnetization is fixed, and a “soft” layer, in which the direction of the magnetization can be switched by application of currents. These currents are applied through conductive write lines referred to as bit lines and word lines, which are disposed so that the bit lines are orthogonal to the word lines. In an MRAM array, an MTJ memory cell is located at each intersection of a bit line with a word line.
To switch the direction of magnetization of the soft layer of a particular cell, currents are applied through the bit line and the word line that intersect at that cell. The direction of these currents determines the direction in which the magnetization of the soft layer will be set. The combined magnitude of the currents through the word and bit lines must be sufficient to generate a magnetic field at their intersection that is strong enough to switch the direction of magnetization of the soft layer.
In some MRAM devices, the bit and word lines may be partially surrounded by a magnetic liner (also called cladding), which serves to focus the magnetic field in the direction of the soft layer. Through use of such a liner, the amount of current that is required to generate a magnetic field sufficient to switch the direction of magnetization of the soft layer is substantially reduced. Additionally, use of a magnetic liner on the word and/or bit lines may partially shield the magnetic field from adjacent memory cells, preventing inadvertent switching of adjacent memory cells.
However, use of magnetic liners for the bit and/or word lines may cause difficulties, because the magnetic liners produce stray magnetic fields originating from their end regions. These stray fields may cause an inhomogeneous offset field over portions of the MRAM array. This could cause, for example, areas of the array that are near the ends of the magnetic liner to require a different current to generate a magnetic field sufficient to switch the direction of magnetization in memory cells, which may cause cells in these regions to be inadvertently switched.
One approach to avoiding the problems associated with stray fields caused by the use of magnetic liners is to extend the liners beyond the region in which MTJ memory cells are present. Using this approach, the areas around the ends of the liner, which are subject to stray magnetic fields, are not used for memory cells. While this will avoid the difficulties associated with use of liners, it also substantially reduces the area of a device that is usable for MTJ memory cells.
Another way in which these difficulties may be avoided is through use of circuitry that offsets the effects of the stray magnetic fields in regions of the MRAM device that are near the ends of the magnetic liners. The stray magnetic fields may be measured, and circuitry added to the MRAM device to compensate for their effects. Of course, this added circuitry will increase the complexity of the MRAM device, and will require space on the device, reducing the space that may be used for MTJ memory cells.
What is needed is a design for magnetic liners for the word and/or bit lines of an MRAM that substantially reduces the stray fields originating at the ends of the liners, without substantially increasing the complexity of the MRAM device, or reducing the area of the device that is usable for memory cells.